Production of magnesia



arch 22, 1949. R. D, PlKE PRODUCTION 0F MAGNESIA Filed Nov. 22, "1944 u m.: Hu.

zuzm G8 E SW3@ INVENTOR .BoB/RT D. PIKE/t MW my Patented Mar. 22, 1949 PRDUCTION F MGNESIA Robert i). `Pike, Pittsburgh, Pa., assignor to Har bison-Walker Refractories Company, Pittsburgh, Pa., a corporation of Pennsylvania Application November 22, 1944Serial N 0.564,628

7 Claims.

This invention relates to the puriiication of magnesium hydrate produced by reaction of calcium hydrate and magnesium chloride, and to thickening apparatus generally useful in connection therewith and for treating other materials.

The reaction of hydrated lime, or calcium hydroxide, [Ca(OH)2l with magnesium chloride (MgCl2) brine to produce solid hydrated magnesia [Mg(OI-I)2l and a calciumchloride (CaClz) brine is Well known. In the practice of the present invention the calcium hydrate may be obtained by the hydration of calcined limestone. However, the present invention is applicable particularly to the production of magnesia from dolomite (CaCO3.l\/igCO3), especially by the processes disclosed and claimed in my copending applications Serial No. 402,935, filed July 18, 1941, now Patent No. 2,373,911, dated April 17, 1945, and Serial No. 504,128, filed September 28, 1943, now Patent No. 2,373,913, dated April 17, 1945, and it may therefore be described with particular reference thereto, by Way of illustration but not of limitation.

In accordance with the process of Patent No. 2,373,911 dolomite is calcined to convert the calcium and magnesium carbonates to oxides, and the calcined material is dry hydrated to produce a large proportion of granular hydrates. Most suitably this is accomplished with a calcium chloride brine of about 1.05 to 1.08 specific gravity that is free, or substantially free, from magnesium chloride, as by mixing the calcined dolomite with about 38 to l0 per cent by weight of such a brine. Advantageously the hydrated material is aged for a period of time after which it is classified to separate it into a granular fraction and a fines fraction.

The word granular as thus used refers to particles larger than about 325 mesh, most suitably larger than about 200 mesh, and not appreciably greater than 65 mesh. There may, of course,

be some overlapping yoi" particle sizes of the magnesium chloride brine to convert the hydrated lime of the dolomite to hydrated magnesia. In this latter step it is important to supply the magnesium chloride in an amount by Weight not less than about or 105 per cent oi the amount stoichiometrically necessary, and as much as per cent excess may be used. The resultant slurry of magnesia in a calcium chloride brine containing the excess magnesium chloride is then settled, suitably in a countercurrent decantation (C. C. D.) system. The underflow from the last of these thickeners is then filtered to produce a cake of hydrated vmagnesia Which is treated as desired, for example by calcining it to produce synthetic dead burned magnesia (MgO).

The magnesium chloride brine used in the foregoing reaction is regenerated in the carbonate branch oi the process by reacting the nes fraction of the hydrated dolomite with the `brine overflow from the C. C. D. thickeners and with carbon dioxide to produce calcium carbonate (CaCOa) and regenerate magnesium chloride brine containing a small amount of calcium chloride. In the carbonate vbranch-of the process of Patent No. 2,373,911 the hydrated dolomite iines are rst subjected to a precarbonating reaction with suiicient of the calcium chloride brine from the aforementionedC. C. D. system to convert all of the hydrated lime in the fines to calcium chloride so that the material passing to the `carbonator comprises a suspension of hydrated magnesia in a calcium chloride brine containing a small amount of magnesium chloride but substantially no hydrated lime. This slurry is then treated with carbon dioxide to produce lcalcium carbonate which is subsequently removed, .as by thickening and subsequent filtration, and the MgCl2 brine recovered is returned to the magnesia branch. Most suitably this latter brine is evaporated, and made up With magnesium chloride `as need be, so that as supplied to the magnesiabranch it has a specific gravity .of about 1.10 to 1.14 and contains about 1l per cent of magnesium .chloride and 3 per cent of calcium chloride.

In this manner I amable to produce a hydrated magnesia that settles quickly and iilters freely, both much more so than magnesia produced in accordance with prior art proposals. Although the principal reactions have been well known for many years, magnesia as produced in accordance with prior art proposals settled slowly and was very difficult to filter and Wash, due primarily to the tendency of the magnesia to be dispersed in Very finely divided form. By observation of the factors set forth above, the process described results in reducing dispersion of the hydrated magnesia and the product is in a form that settles rapidly and filters freely.

The process of my aforesaid application Patent No. 2,373,913 is generally similar but is predicated largely upon my discovery that dispersion of the hydrated magnesia can be suppressed still further by providing the magnesium chloride brine used in the magnesia branch with S03 in solution within certain limitations. The sulfate is preferably present as gypsum, or calcium sulfate (CaSOi). The brine should contain sulfate equivalent to about 0.0015 to 0.0025 gram of S03 per cc. of brine, which is equivalent to about 0.134 to 0.222 per cent of S03. In a typical operation the calcium chloride brine leaving the magnesia branch will contain about 0.0014 to 0.0017 gram of S03 per cc. By aging this brine, gypsum will be spontaneously precipitated so that the brine passing to the carbonate branch will contain only about 0.0006 to 0.001 gram per cc. This is advantageous because the characteristics of the calcium carbonate produced are better than where the brine is fed directly from the magnesia branch to the carbonate branch. Typically, the magnesium chloride brine regenerated in the carbonate branch will contain about 0.000065 to 0.00014 gram of SO: per cc., and gypsum or other appropriate sulfate may be added to it on its way to the magnesia branch to bring the sulfate content within the range just stated.

- In the practice of the process of my aforementioned patent the magnesium hydrate produced may contain as much as 3 per cent of lime (CaO) on the calcined basis. This is satisfactory for many purposes, as for the production of refractories. It would be desirable for some purposes, however, to produce consistently magnesia containing as little as 2 per cent, or even not over about 1 per cent, of lime, while retaining the form of the magnesium hydrate that characterizes the two foregoing processes and results in highly satisfactory and commercially economical rates of settling and ltration.

In practicing the inventions of my aforesaid patents it has been found that the hydrated magnesia actually settles so rapidly that it can notr be handled satisfactorily in conventional forms of thickeners used for countercurrent decantation washing.

Thus, the usual type of thickener comprises a sedimentation tank the bottom of which is provided centrally with a discharge cone into which the thickened product passes and from which it is withdrawn by a pump. Experience with my process has shown that the greater part of the magnesium hydrate settles so rapidly into the discharge cone of the first thickener of the series that it packs and tends to stop up the off-take to the pump, and this is so despite the fact that it is customary to provide a scraper in the cone that moves with the rakes which rotate in the sedimentation tank and move the thickened material toward and into the discharge cone. As the particle size of the magnesium hydrate is not substantially reduced in passing through the series of thickeners, the same thing occurs in each succeeding thickener. The consequence is that with known types of thickeners the C. C. D. system quickly becomes stopped up and inoperative.

A major object of the present invention is to provide a process of producing high purity hydrated magnesia by reaction of calcium hydrate and magnesium chloride that is simple, is eiective to reduce the lime content of the magnesia to not over about 2 per cent, and as little as 1 per cent, on the calcined basis, and which is particularly effective when applied to the processes of my above-mentioned applications.

A further object is to supply such a process in which the reactions between calcium hydrate and magnesium chloride is continued in the thickeners so that even the largest and most stable particles of the creamed hydrated dolomite are substantially completely reacted.

Yet another object is to provide a process of the type mentioned in which the magnesium hydrate is brought into contact with a further amount of magnesium chloride to eiect preferential adsorption of magnesium chloride with displacement of adsorbed calcium chloride, whereby to reduce the lime content of the iinal magnesia product.

Still another object is to provide a thickener that is adapted for use with slurries and the like that contain rapidly settling solids, the discharge well of which does not become clogged, and which is of simple, sturdy and relatively inexpensive construction.

The invention will be described with reference to the accompanying drawing which is a schematic representation illustrative of the preferred embodiment of the process provided by the invention and which illustrates in suicient detail thickeners in accordance with the invention.

The lime content of magnesia as produced in accordance with my above-described processes arises from two sources. First, to avoid dispersion of the granular hydrated magnesia it is desirable to restrict the time during which the precipitated magnesia is in contact with brine. Consequently some of the larger particles of the granular dolomitic hydrates may escape complete reaction and will therefore appear as such in the magnesia. Second, calcium chloride may be adsorbed from the brine by the granular magnesium hydrate; it is difcult, if not impossible, to remove such adsorbed chloride completely by washing of the magnesia lter cake, and this chloride is converted to calcium oxide upon calcining of the magnesium hydrate.

I have discovered, and it is upon this that the present invention is predicated in part, that by subjecting the precipitated magnesium hydrate to longer contact with dilute brines containing magnesium chloride it is possible to convert unreacted lime to calcium chloride, with production of an equivalent amount of hydrated magnesia, and simultaneously to effect desorption of calcium chloride from the magnesium hydrate by preferential adsorption of magnesium chloride from the dilute brine. Thereby I am able to produce magnesia containing, on the calcined basis, not over about 2 per cent of lime as CaO, and even as little as 1 per cent. Most suitably I accomplish this by passing the slurry of magnesium hydrate in calcium chloride brine into the iirst of a countercurrent decantation series of washing thickeners to remove the greater part of the brine, replacing it mainly with water and partly with a brine containing magnesium chloride in greater concentration relative to calcium chloride than the brine with which the magnesium hydrate would be associated if water alone were used for washing.

The invention may be described with further reference to the accompanying drawing which represents the process as applied, by way of illustration, to the production of hydrated magnesia from dolomite. The dolomite is preferably ealeincm. hydrated', and classiiied` into granular and' nes: fractions. in accordancewith my afore= said. patents.. Ilhe granular dol'omiti'c hydrates are stored in a. bin l which feeds a continuous. conveyor H that deliversA the granular' hydrates. to: acreamer t2 where they are. mixed with a cal ciumy chloride brine, most suitably substantially free frommagnesium chloride, to produce athick. and'. lump-free slurry.. This; slurryl passes in a continuous' stream of the. rstof" two. reactors which. preferably take. the form of air-lift'. agi'.- tators.. It; is. preferred. to use. two`A of these.l in series: to minimize short circuiting of' unreacted particles.. In. the first agitator I3 the. creamedv slurryr meets an incoming stream: of magnesiumchloride brine, for example, a. brine of. the: composition and concentration disclosed in Patent No...21,3.'l3,911 and preferably containing' dissolved sulfate as; disclosed. in Patent/No.. 25,373,913. They creamed slurry and brinev arefed suitably in such proportions that` there is formedy in the agitator l-3fa thin slurry containing about 6 per cent, solids which: is lifted by air pressure through the central hollow rotating` shaft 141 and is. distributed through the rotating arms l5 carried by shaft I4..

thickening and washing system show-n. in; this:

instance as. comprising three thickeners.

The thin slurryis fed by pipe tothe first, or

No. l., thickener Where. it entersa. distributor 2l,.

suitably by impinging against. a. centrally posi` tioned cone, as shown, for uniform. distribution. The slurry passes by' gravity from 2|. through. a series` of pipes. 22' which deliver it into a feed well 23, disposed centrally of the sedimentation tank 2:41' which... as usual, has a sloping bottom 25 and is provided with rotatable ra-kes; 2.6;

Inconventional' thickener design the collecting sump; is. ordinarily an inverted frustrum. of a. cone with the pump attached to the lower end,` and the material. which settles, into` the cone is stirred; by means attached to the rake arms. so

that this stirrer' necessarily: operates very slowly.. Such;y conventional; thickeners do not. provide sufcient. agitation in the: sump to keep the magnesium hydrate, or other. rapidly settling or cementitious material, in a fluid condition with the result that the. sump becomes stopped up.

In accordance with the present. invention I provide a sump 2-1- that is preferably cylindrical.

andi is. of greater volume than has been.I custom-v ary; and I provide: it withan independently dri-uen agitator, or stirrer, thatv can be actuated. at sufficient speed to. keepy theV granular settled material in suspension' and in a suiiiciently. fluidcondition that itA does not clog the` sumpv outlet.

'llo thisend rakes 26 aredriven inthe embodi` mentshownby a hollow shaft. 28- through a. gear or. pulleyI 29: at the. very slow speed that is conventional inthe art and necessary in athickener:`

Through shaft 28 there extends an. inner shaft.-

Whichish driven through a second gear or pulley. 3|v andV which carries. at itsA lower endfay Stir-rer 32: is..

stirrer 32L disposed sump. 21.

operated at. a. rate'. which is always greater' than that.' ofi then rakesz 26= to. keep. the; thickened slurry in the sump 2l contnuously'inaud conditionA which prevents. caking and cementing of the hydrates.

In producing hydratedY magnesia byl this procedure I prefer to retain. the. thickened. slurry` in the-i sump of the rst thickener for' from 1'- .toI 3 minutes and to operate the stirrer at' a peripheral velocityv of the blade tips off from 10y to 3ul ft. per minute.v Under these conditionsrof detentionand agitation the reaction between the: mag,- nesium chloride of the brine and any unreacted lime present. is continued' Without material breaking down and dispersion of the granular particles: of magnesium hydrate', and' the? magnesium hy'- drate undergoes a marked preferential adsorpw tion of magnesium chloride in place ofY calcium chloride, provided the ratio of* calcium chloride. to magnesium chloridey is. not over about 2 or: 3-

to l.

The'thickened material is removed from sump. 2l by a pump 33 and delivered' through feed well 34 into the. second, or No.` 2.,. thickener 35, the construction of which is the same as. described. in

connection with the Iirstthickener'. The underflow from this thickener is delivered to the third, or No. 3, thickener 36, which likewise is constructed the same. as. the.- first. two. thickeners.

The underflow from thickener 3.6 is. passed by a. pumpfromsump3l. to anota-ry filter 38-fvvherethe: cake is. washed with. water and subsequently re moved for final treatment..

The combined'. ltrate andi wash water from iilter 38? are passed; to. the: feed; well. 34. of the No. 2' thickener.; The overflow from the. No. .3 thickener likewise is passed to the-No.2 thickener and, similarly,. theV overow from` they No.y 2

thickener goes: to. the: feed; well. of; the No.v l. thickener..

In. ordinary countercurrent decantation. practice. waterv alone.l is. added; to; the: feed welli ofv the No.. 3v thickener.. In accordance. with. the

present. invention, howeven. I' nd. that by sub stitutingt for about 15. percent ofA this Water a.

magnesium chloride brine,.suitably the same as that suppliedto the air-lift agitators, I can re ducer thezlime content ofi the magnesium hydrate to the very loW- limiti. that is characteristic of this invention.. This. I attribute to reactionbetween magnesium chloride and unreacted' lime in. the thickener.' surnps,l and. also. to.l thel factl that by lowering the ratioA of calcium.. chloride; toy mag.- nesium chloride in the. No; 3i thickener. toa very low value by introducing; some magnesiumI chloride brine, MgCl2 is adsorbed'. by the; magnesium hydrate particles in. preference. to CaClz.. The4 quantity ot unreacted lime is. small` and likewise. thev concentration of: magnesiumchloride. but the. conditions. provided: are. just. those which I have found tobe necessary to. remove most of the unreacted lime and adsorbedcalcium chloride.

Ihave described mixing about. S5y per cent of the Wash Water which would, ordinarily be fed to the No.. 3 thickener with. about4 15 per cent ofthe same quantity of magnesium. chloride brine containing abouti l1 per cent. of magnesium chloride and 3 per cent of` calcium chloride,r and of about 1.12 speciicgravity. The. specific/gravity ofthis mixture is. about 1.02. The. same specic: gravity may be had by using' ane appropriately larger amount of Wash. water. from the. carbonate filter in the carbonate.- branch. of. myf abovezide.ntiedI applications.

The following. data. arerepresentatine or the.

7 invention as applied to the production of 100 tons per day of magnesia from dolomite of the Niagara formation of Northwest Ohio:

Item Quantity Unit Granular hydrated dolomite 222 lbs/minute, Magnesium hydrate from #3 thickener... 218 Do. Clgm chloride brine to cream s. g. 41. 4 G. P. M. MgCl; brine, 11% MgCl2, 3% CaCh, 338 Do.

s. g. 1.118 to #1 agitator. Underilow oi #3 thickener average 86 Do. Wash water to #3 thickener 99 Do. Magnesium chloride brine to #3 thick- 20 Do.

ener. Wash water to rotary drum iilter 22 Do. Overflow #l thick 420 Do. S. g. overflow of #l thickener 1.093

gCh 41 rim/liter. Sagl; 81. 8 D0.

a z Mg G12 l. 74 Ratio. S. g. overilow #2 thickener-. 1.046 MgCl, 17. 7 gmJhter. Sagl? 47. 5 Do.

u a q Mgoh 7 Ratio. Bleed to CaCIz branch.--. 75 G. P. M. S. G. overflow #3 thickener 1.027 M C1 13.7 gmJlitcr.

M geh 2. 0 Ratio. Ch content as chloride oi the mag-nesium 2. 39 Percent.

hydrate, calcined basis.

The magnesia produced in that test when completely calcined had a content of about 1.5 per cent of lime. The filtering rate, in terms of the calcined product, was about 90 pounds per s'q. foot of lter area per 24 hours.

By operating in the same manner but omitting the addition of magnesium chloride to the No. 3 thickener, using granular dolomitic hydrates of maximum stability prepared in accordance with Patent No. 2,373,9113, the ratio of calcium chloride to magnesium chloride in the No. 3 thickener was increased to 7.35, and the magnesium hydrate upon calcination contained about 3 per cent of lime and about 2 per cent of chlorine as chlorides. If all of the chlorine were present as calcium chloride, this would correspond to about 1.55 per cent equivalent CaO, leaving about 1.5 per cent actual unreacted lime. As the addition of magnesium chloride to the No. 3 thickener in accordance with this invention reduces the lime content to about 1.5 per cent, its eiect was apparently to remove part of the unreacted lime and part of the adsorbed calcium chloride.

For the production of 100 tons of magnesia per day from the Niagara formation dolomite I prefer to use Dorr air-lift agitators, using two in series as shown, each being about 16 feet in diameter with an air-lift of about 8 to 9 feet high. However, three or more agitators may be used. The No. 1 thickener constructed as described and shown may have a diameter of about 88 feet, while the No. 2 and No. 3 thickeners may have a diameter of about 56 feet. All of the thickeners are suitably 8 feet deep.

Various modications are, of course, permissible. For example, the pipes 22 may be adjusted radially by appropriate means to cause more or less of the solid material to drop into the central sump 21 or onto the sloping bottom 25.

It will be understood that the new type of thickener described herein, and which forms Va part of the invention, is not restricted to the treatment of magnesia slurries but is of general utility where thickening is necessary, especially with rapidly settling cementitious solids.

The calcium chloride supplied to creamer I2 may be derived from any suitable source but in a cyclic process of producing magnesia. from dolomite I prefer to form it in a separate branch of the process as described in Patent No. 2,373,911. Preferably, a portion of the overflow from the No. 2 thickener is bled to a reactor where it meets a portion of the fines from the dolomitic hydrates classication, the two being reacted in amounts such as to convert all of the magnesium chloride in the brine to hydrated magnesia and produce a brine of calcium chloride substantially free from magnesium chloride. The product is subjected to a thickening operation. The underilow from the thickener may be fed to the carbonate branch, as to the precarbonating step, and the overflow is passed to Creamer l2 and to the hydrator of my aforesaid copending application.

According to the provisions of the patent statutes, I have explained the principle and mode of operation of my invention, and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. That method of producing high purity hydrated magnesia which comprises treating hydrated lime with magnesium chloride brine in an amount supplying a large excess of magnesium chloride over the requirement of the reaction and thereby producing a slurry of hydrated magnesia. containing unreacted lime and carrying adsorbed calcium chloride, subjecting said slurry to countercurrent decantation washing in a series of thickeners, introducing into the last of said thickeners an aqueous solution of magnesium chloride in an amount small relative to that used for treating said lime and such that the ratio of calcium chloride to magnesium chloride is low.. and thereby causing the hydrated magnesia topreferentially adsorb magnesium chloride with reduction of its lime content, and recovering the lime-impoverished magnesium hydrate.

2. A method according to claim l, said hydrated lime being supplied as granular dolomitic hydrates, and the ratio CaCl2:MgCl2 in the last washing is not over about 3:1.

3. A method according lto claim 1 in which the ratio of CaCl2:MgCl2 in said thickeners is not over about 3:1.

4. A method according to claim 1 in which desorption of CaClz and reaction of residual Ca(OH)2 is assisted in said thickeners by subjecting the slurry in the thickener sumps to gentle agitation at a rate greater than the rate of agitation in the sedimentation portion of the thickeners.

5. That method of producing high purity hydrated magnesia which comprises treating lime with an aqueous solution containing magnesium chloride to supply a large excess of magnesium chloride over the requirement of the reaction and thereby producing a relatively dilute slurry of magnesium hydrate, settling and recovering a thickened slurry of said magnesium hydrate, and washing the thickened slurry with a dilute aqueous solution of magnesium chloride to supply magnesium chloride in an amount that is small relative to that used for treating said lime and thereby reducing the lime content of the magnesium hydrate.

6. That method of producing high purity hydrated magnesia. which comprises treating lime with an amount of an aqueous solution ot magnesium chloride to supply a large excess of magnesium chloride over the requirement of the reac- UNITED STATES PATENTS tion and thereby producing a relatively dilute Number Name Date slurry of magnesium hydrate, and subsequently 867,958 Dorr Oct. 15, 1907 and separately treating said magnesium hydrate 5 1,739,302 Gregorich Dec. 10, 1929 with a dilute aqueous solution of magnesium 2,124,002 Mastin July 19, 1938 chloride supplying a relatively small amount of 2,224,780 Chesny Dec. 10, 1940 magnesium chloride and thereby reducing the 2,227,534 Collins Jan. '7, 1941 lime content of the magnesium hydrate. 2,233,619 Linch Mar. 4, 1941 7. A method according to claim 6 in which said 10 2,233,641 Ramsey Mar. 4, 1941 lime is treated with said solution of magnesium 2,255,422 Heath et al S6131?- 9, 1941 chloride to supply not less than 100 per cent excess 2,276,245 Clarke Mar. 10, 1942 magnesium chloride over the requirement of the OTHER REFERENCES reaction.

ROBERT D PIKE. 15 Treadwell and Hall: Analytical Chemistry, vol.

REFERENCES CITED I, pp. 46-49, MacM1llan Company, New York, 1929.

The following references are of record in the le of this patent: 

